1. Field of the Invention
The present invention relates to a material supply system, which may be used in a car assembly plant to coat automotive components or works with constant amounts of liquid material such as a sealing compound, or to fill them with constant amounts of liquid material such as an adhesive or grease.
2. Description of the Related Art
Generally, in a car assembly plant, a plunger pump, which is a high pressure pump, sucks liquid material such as a sealing compound or an adhesive from a storage tank and supplies it through supply lines to dispensers, each of which is connected to one of the lines. The dispensers coat or fill works with the liquid material. In such a system, a plunger pump or another high pressure pump is used to supply liquid material to one or more distant places.
FIG. 4 of the accompanying drawings shows a conventional system for supplying a sealing compound or other liquid material from a storage tank 108 to two or more distant dispensers 103, one of which is shown, for coating works with the material. With reference to FIG. 4, a plunger pump 101 is connected with supply lines 102, one of which is shown. Each supply line 102 is connected with one of the dispensers 103.
The supply line 102 consists of a primary supply line 102′ and a secondary supply line 102″, and is fitted with a pressure reducing valve 104. The primary supply line 102′ is upstream of the pressure reducing valve 104 and high in pressure. The secondary supply line 102″ is downstream of the pressure reducing valve 104 and low in pressure. The pressure in the primary supply line 102′ is kept at a high value of about 15 MPa (150 kg/cm2). The secondary supply line 102″ is fitted with an air-operated valve 105 as an on-off valve.
The plunger pump 101 sucks the liquid material from the storage tank 108 and supplies it under high pressure to the supply lines 102, from which it is supplied to the respective dispensers 103. The dispensers 103 discharge the liquid material directly onto the works so as to coat or fill them with constant amounts of liquid material.
The pressure in the secondary supply lines 102″ of the supply lines 102 (the proper supply pressure for the dispensers 103) is low for the following reason. Because the dispensers 103 are mounted on a robot (not shown) or the like, it is preferable that they be small in size, light in weight and able to discharge constant amounts of liquid material. The dispensers 103 may be small-capacity single-shaft eccentric screw pumps. It is necessary that the discharge pressure of the dispensers 103 be very low in comparison with that of the high pressure pump on the supply side. In other words, there is an upper limit to the supply pressure for the dispensers 103.
Each dispenser 103 is fitted with a pressure sensor 106 near its inlet port 103a. This sensor 106 senses the pressure nearly at the inlet port 103a and outputs a pressure signal to an electromagnetic valve 107, which controls the switching operation of the associated air-operated valve 105 according to the sensed pressure. The air-operated valve 105 is closed if the sensed pressure is higher than a set upper limit value, which may be 0.7 MPa. This valve 105 is opened if the sensed pressure is lower than a set lower limit value, which may be 0.3 MPa.
The dispenser 103 intermittently discharges the liquid material. In order to supply the dispenser 103 with a sufficient amount of liquid material every time the dispenser starts discharging the material after it stops discharging the material, it is necessary to keep the pressure in the associated secondary supply line 102″ high to some extent.
Therefore, as soon as the dispenser 103 stops discharging the liquid material, the pressure in the secondary supply line 102″ rises. When this pressure exceeds the upper limit value, the air-operated valve 105 is closed. Thereafter, as soon as the dispenser 103 starts discharging the liquid material, the pressure in the secondary supply line 102″ falls. When this pressure falls below the lower limit value, the air-operated valve 105 is opened. Thus, every time the dispenser 103 starts and stops discharging the liquid material, the pressure in the secondary supply line 102″ falls below the lower limit value and rises above the upper limit value. As a result, the air-operated valve 105 frequently closes and opens. This may wear away the air-operated valve 105 and shorten its life.
The applicant's Japanese Unexamined Patent Publication No. 2002-316081 (para. 0017–0020) discloses a material supply system including a supply device and a dispenser, which is connected to the supply device by a supply line. The supply line is fitted with a pressure reducing valve, an on-off valve and a buffer pump, which is a single-shaft eccentric screw pump. The pressure reducing valve is interposed between the supply device and the on-off valve. The screw pump is interposed between the on-off valve and the dispenser. The operation of the buffer pump and on-off valve is controlled on the basis of the pressure in the supply line between this pump and the dispenser. The use of the buffer pump enables the pressure reducing valve to achieve a larger pressure reduction than in the system shown in FIG. 4. This reduces the pressure acting on the dispenser, and prevents liquid from dripping when the dispenser stops and reverses.
As is the case with the system shown in FIG. 4, however, the on-off valve of the system disclosed in the Japanese publication frequently closes and opens. This may shorten the life of the on-off valve, which is expensive.